Steel sheet and method of manufacturing a steel sheet to protect incline of coat layer

ABSTRACT

Provided are a method of manufacturing an electrically heated steel sheet to protect incline of a coating layer and a steel sheet manufactured by the method. The steel sheet may include an Al-based coating layer including an aluminum-iron intermetallic compound through heat treatment before electrically heating the steel sheet i during a hot forming process such that incline of the coating layer in the electrical heating can be prevented. In particular, the method may include: heat-treating a steel sheet; electrical heating the heat-treated steel sheet; pressing and forming the heated steel sheet; and cooling the formed steel sheet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2017-0124971, filed on Sep. 27, 2017, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method of forming an electrically heated steel sheet and a steel sheet manufactured by the method thereof. The steel sheet may include an Al-based coating layer including an aluminum-iron intermetallic compound that may be formed by heat-treatmenting before electrically heating the steel sheet, thereby preventing the incline of the coating layer in the electrical heating process.

BACKGROUND

Recently, as environmental problems have emerged all over the world, methods for reducing fuel have been found to solve these problems. In order to reduce the fuel consumption, as a solution proposed in the automotive industry, there are improving the efficiency of vehicle engines and reducing the weight of vehicles. Reducing the weight of the vehicle has been provided as a good measure to increase the fuel efficiency of the vehicle. However, if the weight of the vehicle is reduced, the strength and the durability required for the vehicle may not be satisfied. Accordingly, the solving of the problem is becoming the biggest goal of the automobile industry.

For example, the automobile industry has developed various eco-friendly vehicles aiming to reduce carbon dioxide emissions to 95 g/km, which is 27% level compared to the current level until 2021. In addition, in order to satisfy 54.5 mpg (23.2 km/l) of a standard value of the corporate average fuel economy (CAFE) by 2025, automobile manufacturers have been motivated to reduce vehicle sizes and improve fuel efficiency.

Generally, the weight of the material can be reduced in accordance with an increase in the number of parts or an increase in weight. At this time, as a method of reducing the weight, many heat treatment technologies have been applied for high strengthening the material or curing the surface of the material. For instance, a hot forming process has been widely used as a manufacturing method for improving the strength of components used in the vehicle. In particular, in the hot forming process, a steel sheet designed as a heat-hardening type may be heated at a temperature greater than an austenitizing temperature, then rapidly cooled at room temperature, and simultaneously pressed by a mold having a cooling channel to obtain a high-strength low forming component through the shape of a product and a change in structure of the steel.

Therefore, the hot forming method has been applied increasingly in order to reduce the weight of the vehicle, however, due to the characteristic of the hot forming process and high manufacturing cost, it is difficult to apply the hot forming process to various components used in the vehicle. FIG. 1 shows a flowchart of a method of manufacturing a hot-formed steel sheet according to the related art. As illustrated in FIG. 1, a general hot forming method can be performed by a blank steel sheet preparing step (S11), a heating step (S13), a hot forming step (S15), and a laser trimming step (S17) in sequence. The heating step in the related art may be a method of using gas or electricity, that is, an indirect heating source. However, the method of using the indirect heating source may have a problem in not only the low thermal efficiency but also the high cost of installing the heating furnace.

Accordingly, in order to solve the above-mentioned problem, in the related art, the electricity has been directly applied to the steel sheet for hot forming to generate heat by using the self-resistance of the steel sheet. For instance, FIG. 2 shows a flowchart of a method of manufacturing a hot-formed steel sheet through electrical heating according to the related art. As illustrated in FIG. 2, the method may include: a blank steel sheet preparing step (S21), an electrical heating step (S23), a hot forming step (S25), and a laser trimming step (S27) in sequence. The electrical heating step corresponds to a heating method using resistance of the steel sheet itself as compared to a general heating step to improve heat efficiency compared to an indirect heating source.

However, the material generally used in the hot forming method in the related art corresponds to a steel sheet including an Al-based coating layer. However, in the electrical heating step, when the electricity is applied to the steel sheet including the Al-based coating layer, a molten coating material may be inclined by magnetic force generated at the time of applying electricity and the electrical heating is avoided.

In order to solve such a problem, for instance, an electrical heating step by using a steel sheet including a Zn-based coating layer may be provided, however the steel sheet including the Zn-based coating layer has low corrosion resistance and the like compared with the steel sheet including the Al-based coating layer.

Thus, the present invention may be provided to solve the problems of the related art described above, and to improve corrosion resistance and the like by preventing the incline of the steel sheet including the Al-based coating layer.

SUMMARY OF THE INVENTION

In one preferred aspect, provided is a method of manufacturing an electrically heated steel sheet to prevent the incline of a coating layer capable of increasing energy efficiency in a hot stamping process and a steel sheet. As such, the present invention may provide, simultaneously, reducing production costs by heat-treating before electrically heating the steel sheet including an Al-based coating layer.

In further preferred aspect, provided is a steel sheet manufactured by electrical heating that may have improved rust resistance and weldability and reduced cracks by using the steel sheet including the Al-based coating layer.

Other technical objects desired to be achieved in the present invention are not limited to the aforementioned objects, and other technical objects not described above will be apparent to those skilled in the art from the disclosure of the present invention.

An exemplary embodiment of the present invention provides a method of manufacturing an electrically heated steel sheet. The method may include: heat-treating a steel sheet; electrical heating the heat-treated steel sheet; pressing and forming the heated steel sheet; and cooling the formed steel sheet.

The surface of the steel sheet may include an Al-based coating layer.

The Al-based coating layer maybe aluminum-iron alloyed by the heat-treatmenting.

By the aluminum-iron alloying, the coating layer may include an intermetallic compound of Al₁₃Fe₄.

The term “intermetallic compound” as used herein refers to a compound formed with at least two or more metallic elements and optionally one or more non-metallic elements, by specific interaction (e.g., metallic bond or ionic bond) therebetween. Preferred intermetallic compound may exist in a solid state, e.g., crystalline, or solid solution, with ordered arrangements of the constituting elements. The intermetallic compounds may possess differenet properties from the each constituent, so formation of the intermetallic compounds may change novel properties or improve specific properties, e.g., as melting point, brittleness, corrosion resistance, or the like.

An iron content of the coating layer may be about 24 at % or greater.

A temperature of the heat treatment step may be about 600° C. to 660° C.

A time of the heat treatment step may be 5 minutes or greater.

The heat-treatmenting may be performed by a batch annealing furnace (BAF) method.

The pressing and forming the heated steel sheet and the cooling the formed steel sheet may be simultaneously performed.

The method may further include cutting the cooled steel sheet.

The cutting may be performed using a laser.

Another exemplary embodiment of the present invention provides a steel sheet including an Al-based coating layer and the steel sheet may be manufactured by: heat-treating a steel sheet; electrical heating the heat-treated steel sheet; pressing and forming the heated steel sheet; and cooling the formed steel sheet. Preferably, the electrical heating may be applying an electrical current.

The coating layer may be Al-Fe alloyed by the heat-treating.

By the aluminum-iron alloying, the Al-based coating layer may include an intermetallic compound of Al₁₃Fe₄.

An iron content of the coating layer may be 24 at % or greater.

Further provided herein is a vehicle including the steel sheet as described herein.

According to the present invention, a method of manufacturing an electrically heated steel sheet is provided to prevent the incline of the coating layer and to increase energy efficiency in a hot stamping process and simultaneously reducing production costs by heat treatment before electrically heating a steel sheet including an Al-based coating layer. In addition, the steel sheet manufactured by of the method described herein may have improved rust resistance and weldability and reduced occurrence of cracks by using the steel sheet including the Al-based coating layer.

Other aspects of the invention are disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method of manufacturing a hot-formed steel sheet according to the related art.

FIG. 2 is a flowchart of a method of manufacturing a hot-formed steel sheet through electrical heating according to the related art.

FIG. 3 is a graph illustrating an available range of spot welding of an Al-based coating material, an electrical heated GA-coating material, and an electrical heated Zn-Ni-based coating material.

FIGS. 4A-4C are enlarged photographs illustrating rust resistance of the surface of a steel sheet including an electrical heated Al-based coating layer (FIG. 4A), the surface of an electrical heated GA (FIG. 4B), and the surface of a steel sheet including an electrical heated Zn-Ni-based coating layer (FIG. 4C).

FIG. 5A is enlarged photograph of macro cracks formed on the Zn-based coating layer and FIG. 5B is a schematic view of macro cracks formed on the Zn-based coating layer.

FIG. 6A is an enlarged photograph of micro cracks formed on the Zn-based coating layer and FIG. 6B is a schematic view of micro cracks formed on the Zn-based coating layer.

FIGS. 7A-7C are enlarged photographs of the surface of the steel sheet including the Al-based coating layer (FIG. 7A), the surface of GA (FIG. 7B), and the electrical heated surface of the steel sheet including the Zn-Ni-based coating layer (FIG. 7C).

FIG. 8 is a schematic view illustrating the incline of the Al-based coating layer by a magnetic field during electrical heating.

FIG. 9A is an enlarged cross-sectional photograph of an outer portion and FIG. 9B is a central portion of the steel sheet including the Al-based coating layer in which the incline occurs by electrical heating.

FIG. 10 is a graph illustrating states of aluminum and iron.

FIG. 11A is an enlarged cross-sectional photograph of a coating layer before heat treatment of an electrical heated hot forming steel sheet and FIG. 11B is an enlarged cross-sectional photograph of a coating layer after heat treatment of an electrical heated hot forming steel sheet to prevent the incline of the coating layer according to an exemplary embodiment of the present invention.

FIGS. 12A-12B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 400° C. for 10 minutes (FIG. 12A) and after electrical heating (FIG. 12B).

FIGS. 13A-13B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 1 minute (FIG. 13A) and after electrical heating (FIG. 13B).

FIGS. 14A-14B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 2 minutes (FIG. 14A) and after electrical heating (FIG. 14B).

FIGS. 15A-15B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 3 minutes (FIG. 15A) and after electrical heating (FIG. 15B).

FIGS. 16A-16B are enlarged photograph of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 4 minutes (FIG. 16A) and after electrical heating (FIG. 16B).

FIGS. 17A-17B are enlarged photograph of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 5 minutes (FIG. 17A) and after electrical heating (FIG. 17B).

FIGS. 18A-18B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 6 minutes (FIG. 18A) and after electrical heating (FIG. 18B).

FIGS. 19A-19B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 10 minutes (FIG. 19A) and after electrical heating (FIG. 19B).

FIGS. 20A-20B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer without heat treatment (FIG. 20A) and the surface of the steel sheet including the Al-based coating layer after electrical heating (FIG. 20B).

FIG. 21 is a flowchart of an exemplary method of manufacturing an electrically heated steel sheet to prevent the incline of a coating layer according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about.” It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Terms or words used in the present specification and claims, which will be described below should not be interpreted as being limited to typical or dictionary meanings, but should be interpreted as having meanings and concepts which comply with the technical spirit of the present invention, based on the principle that an inventor can appropriately define the concept of the term to describe his/her own invention in the best manner. Therefore, configurations illustrated in the embodiments and the drawings described in the present specification are only the most preferred embodiment of the present invention and do not represent all of the technical spirits of the present invention, and thus it is to be understood that various equivalents and modified examples, which may replace the configurations, are possible when filing the present application.

The present invention relates to a method of manufacturing an electrically heated steel sheet to prevent the incline of a coating layer and a steel sheet manufactured by the method.

In one aspect, provided herein is a method of manufacturing an electrically heated steel sheet that may include: a heat treatment step of heat-treating a steel sheet; a heating step of heating the heat-treated steel sheet; a press-forming step of pressing and forming the heated steel sheet; and a cooling step of cooling the formed steel sheet to thereby prevent the incline of a coating layer.

In an exemplary embodiment, the electrical heating may be performed by applying a current (electric current) to a metallic plate, for example, a steel sheet. As a result, the ductility of the steel sheet may be increased by the heating. Indeed, the method may provide the steel sheet with increased ductility by a press to improve workability and improve strength. The electrical heating step may have an advantage of reducing manufacturing cost in the heating step because thermal efficiency is greater than using indirect heating source in the related art.

Moreover, in the related art, the aluminum (Al)-based coating layer may not be used in the method using electrical heating because the molten Al-based coating layer is inclined toward the center in the electrical heating step. When the Al-based coating layer is inclined to the center of the steel sheet due to the incline of the Al-based coating layer, various problems such may occur, for example, a coating may be peeled at the central portion of the steel sheet, the corrosion resistance of the outer edge portion of the steel sheet is reduced, and the coating material is burned in the forming step.

In order to solve the above problems, in the related art, a zinc (Zn)-based galva-annealed steel sheet (GA) has been applied. The Zn-based galva-annealed steel sheet has a characteristic suitable for electrical heating, but the corrosion resistance and the like may be less than those of the steel sheet including the Al-based coating layer.

FIG. 1 is a flowchart of a method of manufacturing a hot-formed steel sheet according to the related art. As illustrated in FIG. 1, in the conventional method of manufacturing a hot-formed steel sheet in the related art, a gas combustion or electric heating wire may need to continuously opera. For example, the blank steel sheet preparing step (S11), the heating step (S13), the hot forming step (S15), and the laser trimming step (S17) are sequentially performed, the heating time of the heating step is performed for 5 minutes. However, thermal efficiency in the heating step corresponds to 10 to 25%, which indicates that the thermal efficiency is deteriorated. Furthermore, in order to implement the related art, the space for installing the heating device is approximately 4 m×25 m, so a large space is occupied. FIG. 2 is a flowchart of a method of manufacturing a hot-formed steel sheet through electrical heating according to the related art. In order to solve the problem of the related art as depicted in FIG. 1, e.g., deteriorated thermal efficiency and the large installation space, the method for manufacturing the hot forming steel sheet including the electrical heating step as illustrated in FIG. 2 may include a magnetoresistive heating by a magnetoresistive heating device that may operate or stop if necessary. For instance, the heating time in the electrical heating step corresponds to about 20 seconds, the thermal efficiency corresponds to about 60% to 80%, and the space required for installing the magnetoresistive heating device corresponds to 3 m×4 m to be installed at various locations. However, when the method of manufacturing the hot-formed steel sheet including the electrical heating step as illustrated in FIG. 2 is applied to the steel sheet including the Al-based coating layer, the coating layer is likely to be inclined.

Accordingly, in order to prevent incline of the coating, in the related art, a Zn-based coating material has been used. For instance, the Zn-based coating layer made of the Zn-based coating material has a characteristic suitable for electric heating due to a thick oxide layer and a high density. However, in the method for manufacturing a hot stamping steel sheet using an indirect heating source in the related art, the steel sheet including the Zn-based coating layer is not easily heated compared to the steel sheet including the Al-based coating layer, and mechanical properties are deteriorated and the quality of the coating surface is deteriorated.

FIG. 3 is a graph illustrating an available range of spot welding of an Al-based coating material, an electrically heated GA-coating material, and an electrically heated Zn-Ni-based coating material, which are defects occurring in the Zn-based coating layer. When the Zn-based coating material is used, the weldability is less than that of the Al-based coating material. That is, the probability that a spatter is generated due to an oxide layer of a thick zinc oxide (ZnO) is high, and a current range that may be suitably welded is narrow. As illustrated in FIG. 3, the electrical energy heating Zn-based galva-annealed steel sheet has a range of a current capable of spot welding of about 7.2 to about 8.2 kA, and the electrical energy heating Zn-Ni-based coating layer has a range of a current capable of spot welding of about 7.2 to about 8.2 kA. Meanwhile, the Al-based coating material has a range of a current capable of spot welding of about 6.2 to about 8.2 kA such that performing spot welding has a low current compared to the Zn-based coating layer and performing spot welding with a wider range of the current.

However, the Zn-based coating material layer has a problem in that rust resistance is deteriorated compared to the Al-based coating layer. For instance, although the Zn-based coating material maintains the rust resistance in the early stage due to sacrificial anticorrosion, when the Zn-based coating material is used for a long period of time, the rust resistance is deteriorated and the corrosion resistance compared to the Al-based coating layer. FIGS. 4A-4C are enlarged photographs illustrating rust resistance of the surface of a steel sheet by comparing the surface of the steel sheet between an electrically heated Al-based coating layer (FIG. 4A), the surface of an electrically heated GA (FIG. 4B), and the surface of a steel sheet including an electrically heated Zn-Ni-based coating layer (FIG. 4C). The surface of the electrically heated GA and the surface of the steel sheet including the Zn-Ni-based coating layer are excessively corroded, and the surface of the steel sheet including the electrically heated Al-based coating layer is less corroded.

The Zn-based coating material is vulnerable to macro cracks and micro cracks which are caused due to a low melting point compared to the Al-based coating material. For example, the melting point of zinc is about 420° C., whereas the melting point of aluminum is about to 660° C., so the zinc is melted at a lower temperature. As a result, due to the difference in melting point, in the steel sheet including the Zn-based coating layer, liquid metal embrittlement (LME) in which molten metal penetrates through different kinds of solid metal grain boundaries and exhibits brittle fracture even at low stress. FIG. 5B is a schematic view of macro cracks formed on the Zn-based coating layer and FIG. 5A is an enlarged photograph of macro cracks formed on the Zn-based coating layer. As illustrated in FIG. 5A, macro cracks occur in the steel sheet including the Zn-based coating layer 11, and as illustrated in FIG. 5B, when tensile force is applied in both directions based on the macro cracks, a brittle fracture phenomenon occurs. Further, FIG. 6B is a schematic view of macro cracks formed on the Zn-based coating layer and FIG. 6A is an enlarged photograph of micro cracks formed on the Zn-based coating layer 11. As illustrated in FIG. 6A, the micro cracks occur in the steel sheet including the Zn-based coating layer. As such, when the micro cracks occur on the steel sheet including the Zn-based coating layer, the surface of the coating layer is not uniform and the friction force is increased. As illustrated in FIG. 6B, the friction force increases due to the occurrence of micro cracks in the steel sheet including the Zn-based coating layer.

Meanwhile, since the aforementioned Zn-based coating layer is electrically heated, incline of the coating layer does not occur. FIGS. 7A-7C show enlarged photographs of the surface of the steel sheet including the Al-based coating layer, the surface of GA, and the electrically heated surface of the steel sheet including the Zn-Ni-based coating layer. FIG. 7A is an enlarged photograph of a zinc-nickel based coating layer, FIG. 7B is an enlarged photograph of GA and FIG. 7C is an enlarged photograph of the electrically heated surface of a steel sheet including an Al-based coating material. As such, as illustrated in FIG. 7A-7C, the surface of the GA and the surface of the steel sheet including Zn-Ni-based coating layer maintains good because the incline of the coating layer does not occur, whereas the incline of the surface of the steel sheet including Al-based coating layer occurs due to the electrical heating. However, the steel sheet including the Zn-based coating layer is poor in weldability and rust resistance. Accordingly, an object of the present invention is to provide a steel sheet including an Al-based coating layer in which the incline does not occur even at the electrical heating by determining a cause of the incline of the Al-based coating layer.

FIG. 8 is a schematic view illustrating the incline of the Al-based coating layer by a magnetic field during electrical heating. The incline of the Al-based coating layer indicates that the molten coating material may be inclined and solidified at the center of the steel sheet by the electromagnetic force (Lorentz Force). As illustrated in FIG. 8, when the electromagnetic force is applied to one position, a magnetic field is generated around the position and a magnetic force is generated toward the position. The molten coating material is inclined at the position by the magnetic force. However, since the thickness of aluminum oxide (Al₂O₃), which is an oxide layer of the Al-based coating layer, is less than that of zinc oxide (ZnO), which is an oxide layer formed in the Zn-based coating layer, the coating restraint force due to the oxide layer is small, and as a result, the Al-based coating layer is easily inclined at the position which is applied with the magnetic field. In particular, since aluminum oxide forms a dense oxide film having a smaller particle size than zinc oxide to have a low oxygen diffusion rate, the oxide layer is formed to be thin. As a result, the coating restraint force is also small, and thus, the oxide layer is inclined at the position which is applied with the magnetic field. Furthermore, since the density of aluminum is about 2.7 g/cm³, which is less than the density of zinc of about 7.14 g/cm³, the thickness of the coating layer may be formed in greater thickness even when aluminum is coated with the same amount, and thus, the inclined amount is relatively large. Accordingly, the incline of the aluminum coating layer easily occurs compared to the Zn-based coating layer. FIGS. 9A-9B are an enlarged cross-sectional photograph of an outer portion (FIG. 9A) and a central portion of the steel sheet 13 including the Al-based coating layer 11 in which the incline occurs by electrical heating (FIG. 9B). As illustrated in FIGS. 9A-9B, in the steel sheet 13 including the Al-based coating layer 11, since the coating material is inclined at the center of the steel sheet by electrical heating, the thickness of the coating layer is greater than that of the outer edge portion of the steel sheet.

Accordingly, in order to solve the incline of the Al-based coating layer, a method of independently blocking the electromagnetic field of only the coating layer, a method of preventing the melting of the coating layer, and a method of increasing the thickness of the coating oxide layer have been provided. However, since independently blocking the electromagnetic field of only the coating layer is a practically impossible method, the problems are solved by using other methods except for the method.

FIG. 21 is a flowchart of a method of forming or manufacturing the electrically heated hot sheet according to an exemplary embodiment to prevent the incline of a coating layer. The method may include: a heat treatment step (S101) of heat-treating a steel sheet; a heating step (S103) of heating the heat-treated steel sheet; a press-forming step (S105) of pressing and forming the heated steel sheet; and a cooling step (S107) of cooling the formed steel sheet. Preferably, the surface of the steel sheet may include an Al-based coating layer, and the coating layer may be aluminum-iron alloyed by the heat treatment step. Further, by the aluminum-iron alloying, the coating layer may suitably include Al₁₃Fe₄, which is an intermetallic compound, and the iron content of the coating layer may be preferably about 24 at % or greater.

In addition, the temperature of the heat treatment step (S101) may be about 500° C. to 800° C., 550° C. to 700° C., or particularly about 600° C. to 660° C. and the heating time of the heating step may be performed for longer than 1 minutes, longer than 2 minutes, longer than 3 minutes, longer that 4 minutes, or about 5 minutes.

The press-forming step (S105) and the cooling step (S107) may be simultaneously performed. Further, after the cooling step (S107), the method may further include cutting step (S109) of cutting the cooled steel sheet, and the cutting step (S109) may be performed by using a laser.

The method may further include: a heat treatment step before the electrical heating step compared to the related art to prevent the coating layer from being molten by transforming the steel sheet including the Al-based coating layer into an Al-Fe-based intermetallic compound. Particularly, FIG. 10 is a graph illustrating states of aluminum and iron. As illustrated in FIG. 10, the melting point of aluminum is about 660° C., whereas the melting point of the Al-Fe-based intermetallic compound transformed by the heat treatment step, for example, Al₁₃Fe₄ may be about 1100° C. or greater. As a result, the melting point of the coating layer may be increased to be prevented from melting. Further, the Al-based coating material may be transformed into the Al-Fe-based intermetallic compound to thicken the coating oxide layer.

For instance, when the Al-based coating layer is heat-treated, the coating material including the iron of the steel sheet and the aluminum of the coating layer 101 may be diffused and alloyed to form an Al-Fe-based intermetallic compound, e.g., Al₁₃Fe₄. When the iron of the Al-Fe-based intermetallic compound is diffused into the coating layer, the melting temperature of the coating layer may significantly increase, and when the content of iron in the coating layer including the surface layer of the Al-based coating layer may be increased to about 24 at % or greater, even when the electrical heating is performed at a temperature of 1150° C. or less, only the Al-Fe-based intermetallic compound may remain in the coating layer and may not be molten and not liquid. As consequence, the incline of the coating layer may not occur. FIGS. 11A-11B are enlarged cross-sectional photographs of a coating layer before (FIG. 11A) and after (FIG. 11B) heat treatment, respectively, by an electrical heating to the steel sheet 103 to prevent the incline of the coating layer 101 according to an exemplary embodiment of the present invention. As illustrated in FIGS. 11A-11B, the coating layer 101 may be transformed into the Al-Fe intermetallic compound, and the iron may be diffused to thicken the coating layer.

The heat treatment step may be a batch annealing furnace (BAF) method. The BAF method has an advantage that the coil itself of the steel sheet material may be heat-treated to improve the thermal efficiency, and the heat-treating apparatus may not require a large space compared to the related art to be used in various spaces. Further, in the heat-treating step, in order to maintain the surface quality of the coating, the heat treatment may be performed at a melting temperature of the Al-based coating layer or less, for example, at a temperature of about 600 to 660° C. As such, the coating layer may not be inclined when the heat treatment is performed at a temperature of about 600 to 660° C. which the heat treatment step is performed.

In other preferred aspect, provided herein is a method of manufacturing a sheet by electrical heating to prevent the incline of a coating layer and a hot-heated steel sheet manufactured by the same.

The steel sheet by electrical heating according to an exemplary embodiment of the present invention may prevent the incline of the coating layer. The steel sheet may include an Al-based coating layer on the surface of the steel sheet, as the steel sheet manufactured by the aforementioned method. The coating layer may be alloyed from the iron of the steel sheet and the aluminum of the coating material such that the coating layer may include Al₁₃Fe₄ which may be an intermetallic compound. Furthermore, the iron content of the coating layer may be 24 at % or greater.

EXAMPLE

Hereinafter, the present invention will be described in more detail through Examples. These Examples are just to exemplify the present invention, and it is apparent to those skilled in the art that it is interpreted that the scope of the present invention is not limited to these Examples.

In the present invention, in order to confirm whether the incline occurs, the surface of the steel sheet including the Al-based coating layer was treated at a temperature of 600° C. for 1 to 10 minutes and the steel sheet including the Al-based coating layer heat-treated was at a temperature of 400° C. for 10 minutes and then, it was confirmed whether the incline occurred after the electrical heating.

FIGS. 20A-20B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer without heat treatment (FIG. 20A) and the surface of the steel sheet including the Al-based coating layer after electrical heating (FIG. 20B). First, the steel sheet including the Al-based coating layer which was not heat-treated was electrically heated. FIG. 20A shows a surface of the steel sheet including the Al-based coating layer which was not heat-treated and FIG. 20B shows a surface of the steel sheet after electrical heating (FIG. 20B). As illustrated in FIG. 20B, the incline occurred by the electrical heating on the steel sheet including the Al-based coating layer which was not heat-treated.

FIGS. 12A-12B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 400° C. for 10 minutes (FIG. 12A) and after electrical heating (FIG. 12B) and FIGS. 13A-13B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 1 minute (FIG. 13A) and after electrical heating (FIG. 13B). Further, FIGS. 14A-14B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 2 minutes (FIG. 14A) and after electrical heating (FIG. 14B) and FIGS. 15A-15B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 3 minutes (FIG. 15A) and after electrical heating (FIG. 15B). Furthermore, FIGS. 16A-16B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 4 minutes (FIG. 16A) and after electrical heating (FIG. 16B). As a result of the heat treatment under the above condition, as illustrated in FIGS. 12A, 13A, 14A, 15A and 16A, the Al-Fe-based intermetallic compounds were not generated on the surface. Further, as the result of electrically heating of the steel sheet including the Al-based coating layer under the condition, as illustrated in FIGS. 12B, 13B, 14B, 15B and and 16B, the incline of the coating layers occurred in all conditions.

Meanwhile, FIGS. 17A-18B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 5 minutes (FIG. 17A) and after electrical heating (FIG. 17B), FIGS. 18A-18B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 6 minutes (FIG. 18A) and after electrical heating (FIG. 18B), and FIGS. 19A-19B are enlarged photographs of the surface of the steel sheet including the Al-based coating layer after heat treatment at a temperature of 600° C. for 10 minutes (FIG. 19A) and after electrical heating (FIG. 19B). As illustrated in FIGS. 17A-17B, when the heat treatment was performed at a temperature of 600° C. for 5 minutes, an Al-Fe-based intermetallic compound was generated on the surface, and as a result of electrical heating of the steel sheet, there was no incline at a portion where the Al-Fe-based intermetallic compound was generated. Similarly, as illustrated in FIGS. 18A-18B, when the heat treatment was performed at a temperature of 600° C. for 6 minutes, an Al-Fe-based intermetallic compound was generated on the surface, and as a result of electrical heating of the steel sheet, there was no incline at a portion where the Al-Fe-based intermetallic compound was generated. Furthermore, as illustrated in FIGS. 19A-19B, when the heat treatment was performed at a temperature of 600° C. for 10 minutes, an Al-Fe-based intermetallic compound was generated on the surface, and as a result of electrical heating of the steel sheet, there was no incline at a portion where the Al-Fe-based intermetallic compound was generated.

As a result, like the experiment, the temperature of the heat treatment step may be preferably of 600° C. to 660° C., and the time of the heat treatment step may be preferably 5 minutes or greater.

The steel sheet including the Al-based coating layer as described herein may be heat-treated before electrical heating, as compared with the related art, the production cost may be reduced, the weldability may be improved, the rust resistance or the corrosion resistance may be improved, the production cost may be reduced, the improvement of the energy efficiency may be maximized, the manufacturing cost may be reduced low, a manufacturing method may be simplified and the mass production may be achieved easily, stability of the material and longevity may be promoted by improving corrosion resistance, and the application range of the material is extended.

As described above, the present invention has been described in relation to various exemplary embodiments of the present invention, but these embodiments are only illustration and the present invention is not limited thereto. Those skilled in the art can variously modify and change the above description without departing from the scope of the present invention, and the present invention can be variously changed and modified within the technical spirit of the present invention and the equivalents of claims to be disclosed below. 

What is claimed is:
 1. A method of manufacturing an electrically heated steel sheet, comprising: heat-treating a steel sheet; electrical heating the heat-treated steel sheet; pressing and forming the heated steel sheet; and cooling the formed steel sheet.
 2. The method of claim 1, wherein the surface of the steel sheet comprises an Al-based coating layer.
 3. The method of claim 2, wherein the Al-based coating layer is Al-Fe alloyed by heat-treating.
 4. The method of claim 3, wherein the Al-based coating layer comprises an intermetallic compound of Al₁₃Fe₄.
 5. The method of claim 3, wherein an iron content of the Al-based coating layer is of about 24 at % or greater.
 6. The method of claim 1, wherein a temperature of the heat-treating ranges is of about 600° C. to 660° C.
 7. The method of claim 1, wherein a time of the heat-treating ranges from about is about 5 minutes or greater.
 8. The method of claim 1, wherein the heat-treating is performed by a batch annealing furnace (BAF) method.
 9. The method of claim 1, wherein the pressing and forming the heated steel sheet and the cooling the formed steel sheet are simultaneously performed.
 10. The method of claim 1, further comprising: after the cooling, cutting the cooled steel sheet.
 11. The method of claim 9, wherein the cutting is performed using a laser.
 12. A steel sheet comprising an Al-based coating layer, wherein the steel sheet is manufactured by: heat-treating a steel sheet; electrical heating the heat-treated steel sheet; pressing and forming the heated steel sheet; and cooling the formed steel sheet.
 13. The steel sheet of claim 12, wherein the electrical heating is applying an electrical current.
 14. The steel sheet of claim 12, wherein the Al-based coating layer is Al-Fe alloyed by heat-treating.
 15. The steel sheet of claim 14, wherein the Al-based coating layer comprises an intermetallic compound of Al₁₃Fe₄.
 16. The steel sheet of claim 15, wherein an iron content of the Al-based coating layer is about 24 at % or greater.
 17. A vehicle comprising a steel sheet of claim
 12. 